The present invention relates to a frequency-hopping radio LAN (local area network) system, and more particularly to a radio LAN system which prevents overlapping cells from concurrently hopping to the same frequencies to avoid interference.
The frequency hopping is one of spread spectrum communication schemes, and more specifically changes frequencies over time at which information is communicated. In a conventional frequency hopping system, as shown in literature entitled "Digital Mobile Communication System" (pp 98-101, written by Yukiji Yamauchi and published by Tokyo Electric University Press), a hopping pattern for changing frequencies used at regular time intervals is generated by a hopping pattern generator provided in each of a transmitter and a receiver. A frequency synthesizer is controlled by instructions from the generator to change transmitted and received frequencies at regular time intervals.
The hopping pattern generator in the transmitter side generates a substantially random sequence of frequency values which are used to determine the order of hopping frequencies so as to transmit radio waves at the hopping frequencies in the determined order. In order to provide the hopping frequencies in the same order on the receiver side, the receiver side uses a hopping pattern generator for generating the same sequence as that generated on the transmitter side. It should be noted that the hopping pattern is made on the assumption that it is not changed during communications.
In an environment including a plurality of radio base stations such as a multi-cell environment, the possibility of concurrent use of the same frequency is reduced by taking advantage of a random nature of the hopping pattern. However, this reduced possibility is provided only on condition that a very wide band is assigned so as to permit the frequency hopping to a large number of frequencies.
However, assuming that a radio LAN system using a 2.4 GHz band is assigned, for example, a limited band width of 26 MHz, if a band of 2 MHz is assigned to each channel for ensuring a data transmission rate of, for example, 2 Mbps, 13 channels are merely available in the band width of 26 MHz.
Thus, as the transmission rate is to be increased, a less number of channels are available for the frequency hopping. Therefore, if a plurality of radio LAN systems randomly hopped to the channels without considering by any means the operations of other radio LAN systems, the possibility of concurrent use of the same frequency would be increased because of an insufficient number of channels available for the frequency hopping.
To solve the problem mentioned above, JP-A-7-15443, whose priority document is U.S. Ser. No. 052,329, filed on Apr. 22, 1993, (now U.S. Pat. No. 5,394,433) discloses a control system for automated management of frequency-hopping in a radio communication network.
The radio communication network management system described in JP-A-7-15443 includes a plurality of radio LAN base stations each accommodating a plurality of radio terminals and a wireless network manager for accommodating the radio LAN base stations. Each of the radio LAN base stations receives a frequency hopping pattern assigned thereto from the wireless network manager, and monitors the radio communication environment therearound to check whether the same hopping pattern is being used by any other base station. If not used, the radio LAN base station starts a communication with the assigned frequency hopping pattern. After starting the communication, the base station always measures the degree of frequency interference and continues to use the same frequency hopping pattern if the degree of the frequency interference is below a predetermined value. However, if frequency interference exceeding the predetermined value is occurring, the base station modifies part of the frequency hopping pattern, and again checks whether the modified pattern is being used by any other base station. If used, the base station receives a new frequency hopping pattern from the wireless network manager. Conversely, if not used, the base station performs the communication using the modified frequency hopping pattern. The wireless network manager has a plurality of prepared frequency hopping patterns and manages information on whether or not each of the frequency hopping patterns is available to a certain base station, depending on whether a particular frequency hopping pattern is currently in use or whether interference is occurring, and so on. During communications, each of the radio LAN base stations monitors for interference, checks interference, if found, to classify it into local interference, continuous interference and burst-type interference, and takes a suitable action in accordance with the classification, which may be hop insertion/deletion, hopping pattern replacement, hop advance, or the like, to avoid the interference.
The disclosed control system has a problem that the action cannot be taken before interference occurs. However, if interference actually occurs, this will result in communication faults and end up with interrupted data transmission and reception. It is therefore necessary to predict whether frequency interference will occur and previously take a suitable action, if frequency interference is predicted, to prevent communication faults. The above-mentioned control system does not provide any solution to the predictive control.
In addition, the prior art control system has another problem as described below. When a radio LAN base station changes a frequency hopping pattern assigned thereto, radio terminals placed under management of the radio LAN base station also need to change the frequency hopping pattern correspondingly. In this case, however, the change of the frequency hopping pattern may cause the synchronization between the radio LAN base station and the radio terminals managed thereby to go off. For again establishing the synchronization therebetween, the communication is interrupted for an increased time period. Also, in this event, since communication faults due to noise, error and so on may prevent some radio terminal from receiving a hopping pattern changing instruction from the base station, the radio terminal is left unknown about the change of the frequency hopping pattern. As is understood from the foregoing, the prior art control system has left unsolved the problems on the maintenance of the synchronization between a radio LAN base station and radio terminals managed thereby due to a change of a frequency hopping pattern, on the prevention of any radio terminal from failing to receive a hopping pattern changing instruction from the base station, and so on.
The prior art control system has a further problem that if a plurality of radio LAN systems operate in the same area, two or more radio LAN systems may concurrently use the same hopping frequency which causes interference, whereby an amount of data corresponding to a band capacity cannot be transferred.